In a background art, as shown by FIG. 7, a coaxial flexible piezoelectric cable of this kind is constituted by forming an outer electrode 4 at an outer surface of a piezoelectric tube 3 formed with a coaxial flexible piezoelectric member 2 at a surrounding of a core electrode 1 and forming a protective core layer 5 at a surrounding thereof.
A flexible piezoelectric cable is generally polarized as follows.
Reference 1 (Piezoelectric composite material comprising piezoelectric ceramic powder and synthetic rubber, Powder and Industry, Vol. 22, No. 1, pages 50–56, 1990) shows that the coaxial flexible piezoelectric member 2 is polarized by applying high voltage between the core electrode 1 and the outer electrode 4. This is disclosed also in U.S. Pat. No. 4,568,851. A direction of spontaneous polarization of a ceramic particle is aligned in a direction of an electric field by polarization and therefore, a piezoelectric property is provided to the coaxial flexible piezoelectric member 2. The polarization plays an important role in this respect.
However, according to the above-described method, when high voltage is applied between the core electrode 1 and the outer electrode 4, a defect of a small crack, a clearance or the like is present in the coaxial flexible piezoelectric member 2. As a result, high voltage cannot be applied between the core electrode 1 and the outer electrode 4 and therefore, the coaxial flexible piezoelectric member 2 (normally having a length of several hundreds m or more) cannot be polarized. Further, presence of the defect cannot be detected until high voltage is applied between the core electrode 1 and the outer electrode 4, in other words, until finishing the coaxial flexible piezoelectric cable except polarization and therefore, fabrication thereof becomes unstable and yield is deteriorated. Therefore, it has been desired to specify a position at which the defect is present before polarization.
Therefore, the following polarizing methods are conceivable for the flexible piezoelectric cable.
As shown by FIG. 8, there is conceivable a polarizing apparatus arranging the piezoelectric tube 3 formed with the coaxial piezoelectric member 2 at the surrounding of the core electrode 1 at a piezoelectric tube s path portion 61 of a block-like conductor 6 and connecting direct current voltage generating means 9 between the block-like conductor 6 and the core electrode 1 via a lead wire 81 and a lead wire 82 to apply the direct current voltage. Or, as shown by FIG. 9, there is conceivable a polarizing apparatus directly arranging the piezoelectric tube 3 to the block-like conductor 6 and connecting the direct current voltage generating means 9 between the block-like conductor 6 and the core electrode 1 via a lead wire 8 and the lead wire 81 to apply direct current voltage. According to the polarizing apparatus, the coaxial flexible piezoelectric member 2 is arranged at the block-like conductor 6 or the piezoelectric tube path 61 of the block-like conductor 6 and therefore, the block-like conductor 6 is operated as an outer electrode. Therefore, a portion of the coaxial flexible piezoelectric member 2 arranged at the block-like conductor 6 can be polarized by applying direct current voltage between the block-like conductor 6 and the core electrode 1 by the direct current voltage generating means 9.
However, according to the method of the related art, the following problem is posed.
When direct current voltage is applied between the block-like conductor 6 and the core electrode 1 by the direct current voltage generating means 9, a force of attracting the coaxial flexible piezoelectric member 2 and the block-like conductor 6 to each other is generated by an electrostatic force. Therefore, when the piezoelectric tube 3 is moved, a friction force is generated between the coaxial flexible piezoelectric member 2 and the block-like conductor 6 and the piezoelectric tube 3 cannot be moved or even when the piezoelectric tube 3 can be moved, a large force is needed.
The invention is for resolving the problem of the background art and it is an object thereof to provide a polarizing apparatus and a polarizing method of a coaxial flexible piezoelectric cable capable of moving the piezoelectric tube 3 by a small force.
Further, it is an object of the invention to provide a defect detecting apparatus and a defect detecting method for specifying a defect of a coaxial flexible piezoelectric cable before polarization.